The present invention relates to a medical electrode lead employing a tip configuration to hold the distal tip of the electrode in place. The lead is particularly adapted for use with heart pacemakers.
Today there are currently available many forms of electrode leads which are provided with a metallic distal tip which is placed adjacent to excitable tissue, such as the inside wall of the heart. Electric current is supplied to the distal tip through an interconnected insulated electrode wire to stimulate the heart muscle which is in contact with the tip.
One of the main requirements of a pacing electrode is that the electrode tip be maintained in a stable contact with the heart wall for the entire duration of the implant in a patient. This is particularly difficult during the initial acute stage of about 3 or 4 weeks of implantation while fibrotic material is forming about the lead tip. When such contact is not maintained, either acute or chronic (longer term) there is a potential hazard of pacing system failure due to lead dislodgement from one area of the heart to another where it cannot effectively provide for the stimulating and sensing requirements of the pacing system. Lead dislodgement has been known for some time to be one of the most frequent reasons for lead failure. This is particularly true in the hands of less experienced surgeons. In order to stabilize the position of the lead at the heart wall, a number of surgical and mechanical schemes and also various fixating means have been developed over the years to combat and attempt to overcome this problem. One scheme calls for sustaining the lead within the vein and then immobilizing the patient's arm and shoulder to prevent lead movement while fibrosis occurs.
There have been two types of lead tip mechanical fixating means: the first is sometimes called a passive fixating means and the second an active means. The active means is of the screw-in type wherein a physician manually screws or engages a conductive electrode element into the heart wall. The so-called passive means requires no action of the physician beyond insertion of the lead in the normal manner together with placement of the electrode tip in the apex of the ventricle in the conventional manner. In order to stabilize the position of the lead at the heart wall, such things as wedges and/or tines made of thin, very flexible wire, or elastomeric materials, have been used. Such wire, or elastomeric tines and wedges have been intended to entrap or hold the distal end of the lead in the trabeculae of the heart while the fibrotic material forms around it to hold the lead tip in place.
There are a number of geometrical design requirements placed on the passive lead fixating means so that the lead can (a) pass with minimum resistance through even small veins or lead introducers which are used to introduce leads into a vein, (b) be fixated reliably within the trabeculae, (c) minimize the probability of lead entanglement with other cardiac anatomical structures such as heart valves, and (d) have appropriate mechanical characteristics that enable the lead to be removed after acute or even chronic fixation with a minimum of force. The latter requirement is of particular importance in cases where infection develops at the stimulating site, or when the pacing output of the pacemaker no longer meets the requirements and the lead needs to be repositioned or altered. This often occurs when too much fibrotic material builds up and the stimulation threshold undesirably increases. To withdraw a lead after prolonged chronic implant, substantial forces may be required because of complete fibrotic tissue encapsulation of the tip as well as the tines. There is a possibility of tines tearing off and being left in the body.
An example of a prior art device employing truncated cone sections behind the tip (a so-called wedge tip) is disclosed in Thalen's U.S. Pat. No. 4,030,508. An example of thin, flexible wire tines is shown in Chardack's paper entitled "New Pacemaker Electrodes" published in Vol. XVII of the Transactions of the American Society of Artificial Internal Organs, 1971. Another approach employing flexible tines which are located immediately adjacent the electrode tip is shown in U.S. Pat. No. 4,033,357 to Helland. The advent of porous electrodes, which encourage tissue ingrowth, has been an alternative to the tined electrodes as a means for ensuring fibrotic incorporation into the tip to best ensure correct placement against the heart wall. Such porous electrodes have been of the type having a plurality of small holes drilled through the tip as, for example, with the laser, various sponge-metal type tips, or tips comprised of sintered platinum balls, the interstices between which encourage intergrowth with fibrotic material. Some manufacturers have combined porous tips with tines. Another early electrode of the tined elastomer type was the type MIP 135 of Vitatron Medical N.V. of Holland. This electrode consisted of a platinum-iridium electrode tip and a helicallycoiled lead of elgiloy insulated by a tube of silicone rubber. The tip was fitted with a small barbed silicone ring to prevent dislocation in the critical post-operative period. This lead is disclosed, for example, in Biomedical Engineering, Vol. 4, No. 8, Aug. 1959, page 383. Another tined lead proposal is found in the patent to Citron et al No. 3,902,501, dated Sept. 2, 1975. It is believed the Citron et al lead had no commercial counterpart because of the inability to make the embodiment shown in the patent correctly function. A variety of reasons for this difficulty exist but most of them relate to dimensions of parts. For example, the tines had to be cut for use. The sleeve which was arranged to restrain the tines during implantation did not allow orderly release in the heart. The tines were too long and thus tended to be floppy and difficult to put in place.
In another approach, Doring, in U.S. Pat. No. 4,301,815, described a trailing tined electrode wherein tine-like members extended behind the electrode tip shank so that they could be folded within a cylindrical area smaller than the circumference of the lead tip shank during placement. This significantly reduced the trauma associated with implantation of other types of tine leads, for example, of the Helland or Citron et al type. Another improvement by Doring is shown in U.S. Pat. No. 4,409,994 which consisted of including a recessed portion behind the tip shank where tines could be folded into an area determined by a lap joint thus further reducing the introducer size of the trailing tine lead.
All of the above tines have some difficulty associated with removal after the tine members have been encapsulated in fibrotic material. Another particularly distressing difficulty with the tined lead, especially when used as a trial lead is the propensity for the tip to pass through the tricuspid valve with the tines catching in the chordae tendonae--the thin string-like muscles about the valve. The chordae tendonae get caught in the acute angle joint when the tines meet the tip body and the lead cannot be removed. This type of difficulty does not arise with the wedge tip electrode.
It is an object of the present invention to provide an improvement over electrodes of the wedge type with improved insertion, withdrawal, and acute manipulation characteristics. The molded members of this lead have increased facility for folding back when removal from an entrapped position within the trabeculae is desired. It is another object of the invention to provide an improved lead over the wedge type with increased strength in the molded members. Additionally, if a portion is accidentally torn off, molded members are less likely to be lost in the blood stream.
When we discuss entrapment within the trabeculae upon first placing one of the leads in the heart, we wish to convey the concept of an almost loose, but not quite nonrigid interface of the electrode tip molded parts and the trabeculae. In this relationship, the physician is able to sense the entrapment by gentle tugging on the lead.